Equine Encephalitis

Eastern equine encephalitis (EEE) and western equine
encephalitis (WEE) virus infections occur in horses and
other equines in Louisiana. Eastern equine encephalitis
(EEE) is more common and tends to occur in outbreak form.
These diseases are transmitted to horses and humans by
mosquitoes that have fed on infected wild birds. The EEE
virus was first identified in mosquitoes in Louisiana in
1951 near Ponchatoula in Tangipahoa Parish. The virus is
active in horses to some degree every year. EEE virus
attacks the central nervous system of its host. Unvaccinated
horses are particularly susceptible to the infection. The
disease appears within five days after mosquitoes transmit
the virus to the horse. Onsets of clinical signs of EEE are
abrupt, and affected horses die within three days. Signs of
EEE in horses include fever; a sleepy appearance ; some
muscle twitches of the head, neck, shoulder and flank; and a
weak, staggering gait. Affected animals are soon down,
unable to stand. There is no effective treatment. The
fatality rate is 90 percent or higher. An animal that
survives may have permanent brain damage.

Where does EEE come from?

EEE occurs in nature in a wide variety of songbirds. Blood
samples from New Jersey birds, where most of the research on
EEE has been done, indicate that the blue jay, tufted
titmouse, chickadee, catbird and cardinal are most often
infected. Although these birds do not develop the disease,
they maintain high levels of the EEE virus. They are
considered reservoirs in the disease cycle.
~Culiseta melanura~ is the primary mosquito species that
transmits the virus from bird to bird. This mosquito
inhabits marshes and wooded wetlands and rarely feeds on
people or horses. Consequently, the EEE virus normally
exists in a rather secluded environment where it does not
pose an immediate threat to horses or people.

Are emus susceptible to EEE and WEE?

An exotic species sometimes grown in Louisiana, the emu is
susceptible to EEE and WEE. The emu infected with EEE virus
develops a fatal intestinal disease characterized by bloody
diarrhea. Emu owners need to be aware that an infected emu
is a source of virus to be spread by mosquitoes. A person
should not skin an infected emu or contact the bloody stool.
The emu develops a nervous system illness when infected with
WEE virus. Emus should be vaccinated annually for EEE and
WEE.

What causes of EEE outbreak in horses?

There are two possible explanations for EEE outbreaks in
horses, and both are probably responsible in part during
epidemic years. One factor involves Mother Nature, and the
other is related to health management of the horses.
Transmission of EEE from birds to horses and people can
occur when mosquito populations are high. In this situation,
mosquito species other than Culiseta melanura become
involved in the transmission cycle of the EEE virus. Since
these other mosquitoes are more likely to feed on people and
horses, it gives the EEE virus a means of escape from the
bird reservoir and from the more secluded habitats of
Culiseta melanura. Horses are usually the first to become
infected because they are normally housed in rural areas and
they have a much greater exposure to mosquitoes. The
potential for an outbreak of the disease in horses may
correspond to the number of unvaccinated animals present. It
is unusual to see EEE in a horse that has received the
initial vaccine series and timely boosters. During an
out-break, most people have their animals vaccinated but
then fail to follow up with annual booster shots the next
year or so. Two or three years after the last outbreak, the
population of unvaccinated animals has increased
considerably, and another outbreak is possible.

How do horses get EEE?

Horses become infected when a mosquito carrying the EEE
virus bites them.

What time of the year does EEE infect horses?

EEE cases in horses usually begin to appear in mid to late
summer and increase into the fall. In 1998, following a warm
winter, infected horses were seen in south Louisiana
beginning in early spring. The outbreak continued to spread
north during the summer. In Louisiana, outbreaks of EEE in
horses tend to occur at two- to four-year intervals.

What is the best way to protect my horse?

Annual vaccination of all horses and other equines is
recommended. Horses vaccinated for the first time must
receive an injection of EEE-WEE combined vaccine followed by
a second dose of the vaccine three to four weeks later. This
two-dose series is essential to establish an effective
immune response. A booster shot should be given in the
spring each year thereafter. If an outbreak of EEE or WEE
occurs in the area in late summer or fall, a second booster
is advised. Foals should be vaccinated at 4, 5 and 6 months
of age (three doses total) to ensure protection. Another
booster should be given in six months and annually
thereafter. Vaccines are available from various sources.
Proper handling and administration of vaccine are essential
to ensure adequate protection. An overall health program for
horses is recommended. It would include vaccinations
administered by a veterinarian.

What should I do if my horse develops symptoms?

It is important to find out the cause of the illness. Other
diseases and poisoning may have similar symptoms or clinical
signs. The veterinarian may want to obtain blood samples to
send to a diagnostic laboratory to test for EEE. Rabies,
equine infectious anemia (EIA), equine
leucoencephalornalacia (moldy com poisoning), liver failure,
equine protozoan myelitis (EPM), botulism and other
conditions must be considered. If the animal dies, a
thorough autopsy may be necessary to confirm the cause of
illness. In who infection evidence to done, individuals
proximity transmits some cases; removal of the brain for
laboratory analysis may be all that is needed. EEE in horses
does not require a quarantine. It is important to confirm
the diagnosis of EEE so other horse owners can be told of
the need to vaccinate their animals.

Can humans contract EEE directly from infected horses?

No, the EEE virus is not passed from horses to humans by
contact with body fluids or in other ways. Furthermore, it
is proven WNV is not spread from horse to horse or people
either. EEE-infected horses generally do not have enough
virus in their blood to infect mosquitoes. The horse is
described as a ''dead-end host"! EEE is acquired only
from mosquitoes that have fed on infected birds. An infected
horse does not pose a disease threat to its human owners. It
does indicate that the virus is present in the local bird
population and that local mosquitoes are transmitting the
disease.

How do humans get EEE?

Humans can become infected when a mosquito carrying the EEE
virus bites them.

Does EEE represent a serious health threat to humans?

Human cases of EEE are rare, but they can occur during an
outbreak year. The disease produces serious illness, and the
probability of recovery is less than 50 percent. In overt
cases (cases where infection actually develops into
disease), the virus produces an illness that begins with
headache and stiff neck. As the disease progresses, the
patient can fall into coma, with death as a likely outcome.
Recovery is possible, but individuals who recover usually
have brain damage. Children appear to be more susceptible
than adults. The good news is that most humans who are
bitten by infected mosquitoes abort the infection in the
early stages and recover with no evidence that they ever had
the disease. The overt to unapparent ratio of encephalitis
in New Jersey, where most of the research on EEE has been
done, is estimated at one overt case for every 23
individuals bitten by infected mosquitoes. People who live
near acid water swamps and in close proximity to ponds
filled with emergent vegetation are at increased risk from
mosquitoes that transmit EEE.

What is the best way to protect my family if my horse
contracts EEE?

No human vaccine is available for routine use, so avoiding
mosquitoes is the best protection where EEE is present. In
parishes with mosquito control districts, these agencies
should be made alert. Mosquito control personnel are
familiar with the EEE cycle and have the expertise to reduce
the mosquitoes that transmit the disease. Homeowners as well
as horse owners can take measures to reduce the threat of
EEE. First, avoid mosquito-infested areas and use insect
repellent when exposure is unavoidable. Eliminating
water-holding containers from your property (buckets, tires
and other receptacles) will reduce mosquito breeding in the
immediate vicinity. Horse troughs provide excellent mosquito
breeding habitat and should be flushed at least once each
week to reduce mosquitoes near the paddock area. The use of
residual insecticides for treating mosquito-resting areas
around homes and livestock premises is helpful, too. In some
cases, removal of the brain for laboratory analysis may be
all that is needed. EEE in horses does not require
quarantine. It is important to confirm the diagnosis of EEE
so other horse owners can be told of the need to vaccinate
their animals.